Mutagen-sensitive mutants defective in DNA repair mechanisms have been collected in Drosophila melanogaster and characterized cytogenetically in order to gain a basic understanding of the genetic control of sensitivity to mutagenic agents. The tests used in the initial characterization of these mutants include genetic and cytogenetic mapping, complementation analysis, tests for sensitivity to unrelated mutagens, and tests for pleiotropic effects on related functions such as recombination. A genetic fine structure map of the mei-4l region has been constructed using several independently isolated alleles. This map confirms the large size of mei-41 found during mutational analysis. The mei-41 locus is estimated to cover approximately 300 kilobase pairs of DNA. A nuclease has been identified and is being purified that is under the genetic control of mei-41. This nuclease is antigenically related to nucleases from yeast and Neurospora.